The invention relates to a method for operating a nitrogen oxide storage-type catalytic converter of an internal combustion engine, particularly of a motor vehicle.
In current automotive engineering spark ignition engines as internal combustion engines with direct gasoline injection instead of a conventional intake manifold injection are preferred, since these internal combustion engines, compared to conventional spark ignition engines, have distinctly more dynamics, are superior with respect to torque and output, and at the same time make possible a reduction in fuel consumption by up to 15%. This makes possible so-called stratification in the partial load range in which an ignitable mixture is required only in the area of the spark plug, while the remaining combustion chamber is filled with air. As a result the engine can be operated unthrottled; this leads to reduced load changes. In addition, the direct gasoline injector benefits from reduced heat losses since the air layers around the mixture cloud insulate toward the cylinder and cylinder head. Since conventional internal combustion engines, which work according to the intake manifold principle, as such a high air excess as prevails in direct gasoline injection can no longer be ignited, in this stratified charging mode the fuel mixture is concentrated around the spark plug which is positioned centrally in the combustion chamber, while in the edge areas of the combustion chamber there is pure air. In order to be able to center the fuel mixture around the spark plug which is positioned centrally in the combustion chamber, a concerted air flow in the combustion chamber is necessary, a so-called tumble flow. In the process an intensive, roller-shaped flow is formed in the combustion chamber and the fuel is injected only in the last third of the upward motion of the piston. By the combination of the concerted air flow and the special geometry of the piston which has for example a pronounced fuel and flow depression, the especially finely atomized fuel is concentrated in a so-called “mixture ball” ideally around the spark plug and reliably ignites. The engine control provides for the respectively optimized adaptation of the injection parameters (point of injection time, fuel pressure).
These internal combustion engines can therefore be operated in lean operation for a correspondingly long time; this benefits fuel consumption overall, as has been described in the foregoing. This lean operation however entails the disadvantage that the nitrogen oxides (NOx) cannot be reduced in the lean exhaust gas of a 3-way catalytic converter. In order to keep the nitrogen oxide emissions within the scope of prescribed limits, for example of the Euro-IV limit value, nitrogen oxide storage catalytic converters are generally used in conjunction with these internal combustion engines. These nitrogen oxide storage catalytic converters are operated such that the nitrogen oxides produced by the internal combustion engine in the first operating phase as the lean operating phase are stored in the nitrogen oxide storage catalytic converter. This first operating phase or lean operating phase of the nitrogen oxide storage catalytic converter is also called the storage phase. As the length of the storage phase increases, the efficiency of the nitrogen oxide storage catalytic converter decreases; this leads to a rise in nitrogen oxide emissions downstream of the nitrogen oxide storage catalytic converter. The reduction in efficiency is caused by the increase of the nitrogen oxide fill level of the nitrogen oxide storage catalytic converter. The rise in nitrogen oxide emissions downstream of the nitrogen oxide storage catalytic converter can be monitored, and after a predeterminable threshold value is exceeded, a second operating phase of the nitrogen oxide storage catalytic converter, a so-called discharge phase, can be initiated. During this second operating phase a reducing agent can be added to the exhaust gas of the internal combustion engine and it reduces the stored nitrogen oxides to nitrogen and oxygen. The reducing agents are generally hydrocarbons (HC) and/or carbon monoxide (CO) which can be produced in the exhaust gas simply by a rich setting of the fuel/air mixture. Towards the end of the discharge phase most of the stored nitrogen oxide is reduced and less and less of the reducing agent which can reduce the nitrogen oxide to oxygen and nitrogen comes into contact with the nitrogen oxide. Towards the end of the discharge phase the proportion of the reducing agent in the exhaust gas downstream of the nitrogen oxide storage catalytic converter therefore rises. By corresponding analysis of the exhaust gas downstream of the nitrogen oxide storage catalytic converter, for example by means of an oxygen sensor, the end of the discharge phase can be initiated and it is possible to switch back to the lean operating phase. In known nitrogen oxide storage catalytic converters this switching is carried out at time intervals of for example from 30 to 60 seconds, regeneration, i.e., the discharge phase, lasting approximately 2 to 4 seconds.
To determine a quality factor for evaluation of the storage capacity of a nitrogen oxide storage catalytic converter, WO 02/14658A1 discloses a process in which during the storage phase the raw nitrogen oxide mass flow upstream of the nitrogen oxide storage catalytic converter and a nitrogen oxide mass flow downstream of the nitrogen oxide storage catalytic converter are determined and the state of the nitrogen oxide storage catalytic converter is determined from the two determined values for the raw nitrogen oxide mass flow upstream and the nitrogen oxide mass flow downstream of the nitrogen oxide storage catalytic converter. For this purpose, the two determined values for the raw nitrogen oxide mass flow upstream and the nitrogen oxide mass flow downstream of the nitrogen oxide storage catalytic converter are each integrated over a predetermined time interval and the state of the nitrogen oxide storage catalytic converter is determined by the quotient from the integrated values for the raw nitrogen oxide mass flow upstream and the nitrogen oxide mass flow downstream of the nitrogen oxide storage catalytic converter. In this way, the quality factor is obtained which enables a conclusion about the storage capacity of the nitrogen oxide storage catalytic converter with respect to catalytic converter aging by sulfur poisoning and thermal damage or the ageing-induced decrease of the storage capacity. In particular, in this way the degree of poisoning of the catalytic converter with sulfur will be determined and thus the sulfur content will be corrected in the control device of the internal combustion engine in order to optimize sulfur regeneration. This is because the sulfur which is contained in fuels leads to poisoning of the storage catalytic converter, i.e., to permanent storage of the sulfur in the storage catalytic converter which reduces the storage capacity for the nitrogen oxides. In the nitrogen oxide storage catalytic converter the nitrogen oxides are stored in the form of nitrates, while the sulfur is stored in the form of sulfates. Since the sulfates are chemically more stable than the nitrates, the sulfate cannot break down in nitrogen oxide regeneration. Only for catalytic converter temperatures above 650° C. under reducing conditions can sulfur discharge can be achieved. These high catalytic converter temperatures are however generally not reached in urban driving so that mainly in city traffic does creeping attachment of sulfur in the nitrogen oxide storage catalytic converter occur, which leads to ageing of the nitrogen oxide storage catalytic converter. This ageing must therefore always be considered in the design and operation of a nitrogen oxide storage catalytic converter in order to ensure that catalytic converter ageing over the intended service life of the catalytic converter leads to adherence to predetermined exhaust gas limit values with respect to nitrogen oxide emissions for an aged nitrogen oxide storage catalytic converter. A generic process for operating a nitrogen oxide storage catalytic converter of an internal combustion engine of a motor vehicle is in general already known, in which the nitrogen oxides which have been produced by the internal combustion engine are stored in the nitrogen oxide storage catalytic converter in the first operating phase (lean phase) as a storage phase for a specific storage time, and in which, after expiration of the storage time at a specific switching instant for a specific discharge time, switching to the second operating phase as the discharge phase takes place, in which the nitrogen oxides which were stored during the storage time are discharged from the nitrogen oxide storage catalytic converter. The switching instant in the storage phase is determined as a function of the nitrogen oxide slip as the difference between the nitrogen oxide mass flow which has flowed into the nitrogen oxide storage catalytic converter and the nitrogen oxide mass flow which has flowed out of the nitrogen oxide storage catalytic converter, each relative to the storage time.
Specifically, in order to be able to adhere to the predetermined exhaust gas limit value over the entire service life of the nitrogen oxide storage catalytic converter, in such an operating mode the number of discharges must be matched to the amount of nitrogen oxide which has been discharged per charging and discharging cycle, such that for the storage capacity of an aged nitrogen oxide storage catalytic converter which has been reduced compared to a new nitrogen oxide storage catalytic converter, the amount of nitrogen oxide which has been released during the exhaust gas test time interval does not exceed the predetermined exhaust gas limit value. This amount of nitrogen oxide release which is predetermined per charging cycle for an aged storage catalytic converter is an absolute quantity and constitutes the absolute nitrogen oxide slip, i.e., that as soon as the storage catalytic converter is charged with this amount of nitrogen oxide, discharge takes place. This absolute nitrogen oxide slip as a fixed value applies both to the new and also the aged nitrogen oxide storage catalytic converter.
Since a rich mixture of lambda greater than 1 per discharge is required, with an increasing number of discharges in the course of ageing of a storage catalytic converter, the fuel consumption also rises compared to that of a new storage catalytic converter.
The object of the invention is therefore to make available a process for operating a nitrogen oxide storage catalytic converter with which the fuel consumption especially for new storage catalytic converters can be reduced.